Is there a way to change scrollbar width?
One way to scale scrollbars is... to scale everything: since 2.79, you can scale the UI through a global factor (default to 1). This is useful on hidpi displays, but it could help you too:

by default, the UI as 1.0 scaling factor:



but you can scale it down enough to get all the UI (and also scrollbars) slimmer:



of course now everything is too small, but in Blender you can also scale each area/panel (CTRLMMB drag): of course par of the UI (menu and icons) cannot be resized in this way, but you can at least find a compromise...